1. Field of the Invention
The present invention relates to a television tuner being capable of receiving both an analog signal and a digital signal.
2. Description of the Related Art
FIG. 5 illustrates a conventional television tuner. An analog television signal and a digital television signal are input to a mixer 4 sequentially through an input tuning circuit 1, a high frequency amplifier 2, and an interstage tuning circuit 3. These television signals are mixed with a local oscillating signal supplied from an oscillator 5 to the mixer 4 and are converted into an intermediate frequency signal. Here, the intermediate frequency signal based on the analog television signal is a first intermediate frequency signal and the intermediate frequency signal based on the digital television signal is a second intermediate frequency signal.
The output side of the mixer 4 is connected to an inductance element 5a, and the inductance element 5a is connected to an analog intermediate frequency circuit 13 for processing the first intermediate frequency signal via a first switch diode 6. Similarly, the inductance element 5a is also connected to a digital intermediate frequency circuit 14 for processing the second intermediate frequency signal via a switch diode 7. The first switch diode 6 and the second switch diode 7 are controlled such that the operations of ON/OFF of the two diodes are opposite to each other. In addition, the connection point between the first switch diode 6 and the analog intermediate frequency circuit 13 is grounded by a first capacitive element 8b, and the connection point between the second switch diode 7 and the digital intermediate frequency circuit 14 is grounded by a capacitive element 9b. 
In this construction, upon receiving the analog television signal, the first switch diode 6 is turned on, and the first intermediate frequency signal output from the mixer 4 is input to the analog intermediate frequency circuit 13 via the first switch diode 6. At this time, a low pass filter for attenuating the intermediate frequency signal of an adjacent channel is formed by the inductance element 5a and the first capacitive element 8b. Further, since the second switch diode 7 is in the OFF state, the backward interference signal output from the digital intermediate frequency circuit 14 is not input to the analog intermediate frequency signal 13.
On the other hand, upon receiving the digital television signal, the second switch diode 7 is turned on, and the second intermediate frequency signal output from the mixer 4 is input to the digital intermediate frequency circuit 14 via the second switch diode 7. At this time, a low pass filter for attenuating the intermediate frequency signal of an adjacent channel is formed by the inductance element 5a and the second capacitive element 9b. Further, since the first switch diode 6 is in the OFF state, the backward interference signal output from the analog intermediate frequency circuit 13 is not input to the digital intermediate frequency signal 14. (For example, refer to Japanese unexamined Patent Publication No. 2001-111905 (FIG. 1))
Since the intermediate frequency signal based on the analog television signal and the intermediate frequency signal based on the digital television signal are input to the analog intermediate frequency circuit 13 and the digital intermediate frequency circuit 14 via the first switch diode 6 and the second switch diode 7, respectively, the respective intermediate frequency signals are distorted by non-linear property of the switch diode.
Further, since only the low pass filter is inserted between the mixer 4 and the analog intermediate frequency circuit 13 or the digital intermediate frequency circuit 14, the transfer characteristics suitable for the respective intermediate frequency signal cannot be obtained.